The reference "Elektronik 6/25 March 1983, pages 111 ff." has disclosed a "concept for a press optimization system"60 in which different types of press control systems are described. In this case, the press control system is designated as a complex automation system in which a multiplicity of function groups have to be subjected to control. A hydraulic pump is provided in a pressure network in this case for the purpose of controlling a hydraulic press, piston/cylinder units being used to drive the press ram and their pressing and operating pressures being controlled by means of proportional valves. In this case, the downward and upward movement of the press ram is accomplished via pistons which can be pressurized on both sides. A complicated valve control system takes over the flow of the hydraulic medium in the pressure network.
In conventional hydraulic presses, the unloaded press ram is moved downwards and upwards by means of a separate rapid-traverse cylinder. The actual operating cylinders for applying high ram forces are therefore used only during the actual machining operation, for example during the forming or cutting of a workpiece. In this case, the press operates by means of pressure control in the pressure network, that is to say the pressure in the operating cylinders is substantially increased in order to carry out the operation on the workpiece, this being accompanied by an impressed volumetric flow, that is to say an approximately constant volumetric flow.
Conventional systems having an impressed volumetric flow react to load fluctuations on the press ram with a change in the operating pressure. Overall, a pressure increase in the pressure network therefore effects a compression of the oil column, so that given the relatively high compressibility of the oil column it is necessary to feed a volume of oil before a further pressure increase and thus a continuation of movement can take place. This compressibility of the oil column is also termed "hydraulic spring". This leads to negative oscillation responses in the pressure network.
This means that, due to the fact that they have a high hydraulic volume which is to be displaced, hydraulic presses are, rather, slow and affected by relatively large losses, since the hydraulic medium has to be transferred from low pressures to very high pressures. Pressure losses during the expansion of a respective cylinder chamber for the purpose of carrying out a directed movement can be compensated only partially.
A drive concept for hydrostatic drives having a so-called "secondary control" has been disclosed in the reference MANNESMANN REXROTH: "Hydrostatische Antriebe mit Sekundarregelung (Hydrostatic drives having secondary control), volume 6, pages 13-18, Der Hydraulik Trainer, 8/89". In this case, the "secondary control" are systems having "impressed pressure", that is to say the drive of machines is performed according to the principle of hydrostatic drives, in accordance with which a medium is brought to a higher energy level and then can perform work via a suitable structural device. For example, it is possible in a closed circuit for a hydraulic drive to use an electrically driven feed pump for the pressure medium to transfer the latter to a higher pressure level and to drive a hydraulic pump for conversion into mechanical energy. In an open system, a piston/cylinder unit having pistons which can be pressurized on both sides can be driven on both sides in each case by a driven feed pump via a proportional valve control system.
The "secondary control" described in this reference therefore behaves in a manner similar to an electric DC motor, in which the supply voltage is constant and load variations are compensated via a variation in current. Similarly, in the case of a drive having secondary control the system pressure is held constant and the volumetric flow is kept variable in the event of load variation.
The reference provides no information as to how such a secondary control for a press control system of a hydraulic press can be used.